Introduction

ALFA--Active Logic For Agents--is a reasoner based on Alma/Carne
with representations for beliefs, observations, desires, intentions, expectations
and achievements. In general, an agent based on ALFA can have desires
to perform concurrent actions or achieve concurrent
results at different times.

For a UIA, the actions are:
send commands to TOS, provide information to the user and
update own knowledge base; the results are: readings from TOS
and information from the user. The UIA actions listed in
Section 1.3.9 correspond to performing these actions and receiving
these results. In some cases, the results may directly map to the effect
of an action, but that is not a necessary condition. In other
words, the UIA can have desires to achieve results that are not
effects of known action(s) .

ALFA transforms achievable desires to intentions and achievable intentions to actions. Actions may cause changes to different properties of domain objects and these properties need to be observed to determine whether the changes have been effective. For this, ALFA creates expectations regarding the new value of the property, and these expectations may then get transformed into desires to actually observe the value of that property. For instance, performing the action--turning the heater on--causes an expectation that the water temperature increases; this expectation causes a new desire to observe the water temperature. Thus, ALFA makes use of expectations to determine whether the action that the agent initiates produces the requisite outcome or result. Such expectations can act as desire creators by triggering the formation of desires to observe the result specified in an expectation.

Agents based on ALFA can have the following 6 broad categories of desires or intentions:

to perform simultaneous actions.

to perform simultaneous actions repeatedly.

to observe the values of various properties of objects.

to continuously observe these values.

to achieve specific values for different properties of objects.

to maintain the properties of object at specific values.

In ALFA, a desire for , where is a set of either
concurrent actions or simultaneous results, is represented as
. Here, distinguishes
different desires and is a constant--, , or
--depending on whether denotes a set of actions to be
performed concurrently once, a set of actions to be repeatedly
performed simultaneously, a set of results to be obtained together
once or a set of results to be repeatedly obtained simultaneously,
respectively. and denote the time period in which the
actions are to be performed or the results need to be obtained. The
different values that and can take include all
non-negative integers. Intentions are represented likewise, as
, where the interpretation of
the different variables is the same as that for desires. An
expectation for a result is represented as
, where the variables and
together denote the time period in which an observation of is
expected. Depending on the values of and , a desire,
intention or expectation can be associated with an interval (when
) or point in time (when ).

ALFA represents the last observed value of a property of
an object as
where denotes the time at which the
last observation was obtained.

Preconditions for actions and results are represented as
where is either an action or a result and is either
or
, where is any formula. By assigning
to , it can be specified that the action should precede .

ALFA represents the effect of a set of concurrent actions as
, where is either
or
depending on whether causes a change in the property of an object or an observation of a property of an object. and together specify the duration for which the result can be observed; that is, can be observed for steps, starting steps after the step at which the actions in have been invoked. The value, , indicates that the effect persists.

ALFA represents opposing results and effects as
where and are two possible values for the property of object and opposes (has an opposing effect to) . Examples of and are and .

ALFA marks desires and intentions as
or
where is the name of the desire formula
or intention formula, based on whether the desire or intention is
achievable or not. Only achievable desires get transformed to
intentions; similarly, only achievable intentions get transformed to
actions. ALFA also marks achieved intentions as
and achieved expectations as
so that they don't take part in further reasoning.

When ALFA has an intention to achieve a set of results, it notes a
feasible set of concurrent actions that can cause each result as
where is a unique
identifier that corresponds to the intention, is a result,
is a set of concurrent actions that can produce ,
is the first time step after the actions are initiated
when the result can be observed and is the duration for which the
result can be observed.